Color electrophotographic method and apparatus

ABSTRACT

According to a conventional one-time transfer type color electrophotographic method, since used is a color toner by which the maximum image density can first be obtained by overlapping of 4 to 6 layers, there are problems that the color tone of a mixed color image formed by piling up color-different toner images one after another varies and color irregularity occurs. On the other hand, according to the present invention, by using a color toner whereby the maximum image density of the toner can be attained with one toner layer, the aforementioned problems can be eliminated, and the reproduction of the mixed color image can be achieved by the one-transfer system. In addition, according to the present invention, it is possible to reproduce a clear full-color image with gradation.

TECHNICAL FIELD

The present invention relates to a color electrophotographic method andapparatus which performs an image-exposure on a toner image held on aphotoconductor to form a toner image on the first-mentioned toner imagewith a toner different in color from the first-mentioned toner image.

TECHNICAL BACKGROUND

Conventionally known is a three-times transfer type colorelectrophotographic method in which, using powder toners with threecolors, i.e., yellow (Y), Magenta (M) and cyan (C), overlaps tonerimages with three colors on a transfer sheet by, for each toner,repeating three times an electrophotographic process including charge,color-separation exposure, development, transfer and cleaning.

In this system, it should be required to transfer on a transfer sheeteach of three-color toner images to be successively formed on thephotoconductor without occurrence of position slippage. This provides aproblem that a transfer drum in addition to the photoconductor isrequired to cause the apparatus to increase in size and become complex.

Accordingly, in order to remove the aforementioned problem are proposedvarious one-time transfer type color electrophotographic methods whichdo not require the transfer drum. That is, this is a system in which atoner-image forming cycle including electrification, exposure anddevelopment is repeated several times so as to form on thephotoconductor a plurality of toner images which are different in colorfrom each other before collectively transferring them to a transfersheet. One well known example of such systems is disclosed in U.S. Pat.No. 4,599,286.

For such a one-time transfer system, one important point for determiningthe color image quality is to perform electrification, exposure anddevelopment on the photoconductor having a toner image so as tofaithfully form a toner image on the first-mentioned toner image withrespect to an optical image, the toner image to be formed beingdifferent in color from the first-mentioned toner image. Therefore, theoptical characteristic of a toner layer making up the toner imageresults in being extremely important.

However, since performance required for the toner layer is not knownconventionally, it is difficult to obtain a clear full-color image.Particularly, there are problems that the color tone of a mixed colorimage formed by overlapping of a color-different toner image varies andnonuniformity occurs in color, thereby causing extreme deterioration ofthe image quality of a full-color image.

A description will be given hereinbelow in terms of the problems. In thedevelopment process of the conventional one-time transfer system, asdisclosed in the above-mentioned U.S. Pat. No. 4,599,286, thetwo-component magnetic brush developing method is general where atwo-component developer, being a mixture of a carrier magnetic powderand a toner, is used and the development is effected with the developerbeing spiked by a magneto. A description will be made with reference tothis two-component magnetic brush developing method in terms of the caseof forming a mixed color image in which a first toner image isoverlapped with a second toner image whose toner layer thickness isconstant.

When a photoconductor holding the first toner image is charged and thenthe first toner image is exposed from the upper side with a secondoptical image with constant quantity of light and developed with asecond toner, the adhesion amount of the second toner varies inaccordance with the toner layer thickness of the first toner image. Thatis, in order to obtain a desirable mixed color image, it is essentiallyrequired that the adhesion amount of the second toner is constantirrespective of the toner layer thickness of the first toner image.However, the second toner is adhered to be relatively thick at a portionthat the toner layer thickness of the first toner image is small. On thecontrary, the second toner is adhered to be relatively thin at a portionthat the toner layer thickness of the first toner image is great.

According to analysis of this cause, it has been found that the adhesionamount of the second toner is decreased exponentially in accordance withincrease in the toner layer thickness of the first toner image, morespecifically the number of the laminated toner layers. This is due tothe fact that the scattering quantity of light passing through the tonerlayer increases as the number of toner layers of the first toner imageand hence the quantity of light passing through the toner layer andreaching the photoconductor exponentially decreases to cause the surfacepotential of the photoconductor to vary. For example, in the case of ayellow toner with an average particle diameter of 10 μm, when thequantity of light transmitting one layer (toner covering rate: 50%) istaken as 100%, two layers result in about 20% and three layers result inseveral %. Furthermore, when the photoconductor is charged by means of acorona charger in order to form a second toner image, the chargedamounts of the first toner and the photoconductor are distributed ininverse proportion to the electrostatic capacities of the first tonerlayer and the photosensitive layer. Therefore, in cases where the tonerlayer thickness of the first toner image is nonuniform, it is understoodthat the charged amount of the photoconductor itself becomes nonuniform.

Accordingly, in order to attain a clear mixed color image with theone-time transfer system, it is required to use a toner which canprovide a high density irrespective of a small number of layers andfurther employ a developing method which is capable of performing thedevelopment so that the toner layer thickness is small and uniform.

However, conventional color toners are arranged so that the maximumdensity can be first obtained with overlapping of four to six layers. Inaddition, in the two-component magnetic brush developing method, theadhesion amount of a toner to be developed varies delicately due tovariation of the mixing ratio of the toner and carrier and others evenif the surface potential of the photoconductor is constant, andtherefore, it is extremely difficult to develop the toner layer to bethin and uniform. Moreover, with respect to a high density image inwhich the toner adhesion amount becomes great, the toner layer thicknessvaries so as not to become constant.

From the above-described reasons, when reproducing a high-density mixedcolor image with the conventional one-time transfer system, theone-color toner layer thickness becomes great and the toner layerthickness greatly varies, and therefore, the color tone may vary andcolor nonuniformity occurs.

DISCLOSURE OF THE INVENTION

The present invention eliminates the problems inherent to theconventional systems and provides a color electrophotographic method andapparatus which is capable of attaining to a clear mixed color imagewith high density and stable color tone.

The present invention is characterized in that in a colorelectrophotographic method including the steps of performing an imageexposure through a toner image with respect to an electrostatic imageholding device carrying the toner image and of performing development onthe above-mentioned toner image with a toner different in color from theabove-mentioned toner image, a clear color image with high density andcolor stability can be obtained by using a transparent color toner whichsubstantially provides the maximum density of the toner, determined inthe apparatus, by one toner layer.

According to the present invention, the maximum density of the toner canbe obtained with one toner layer, where it is possible to arrangeone-color toner image with one toner layer. Therefore, since it ispossible to thin the toner layer thickness in forming a high-densitymixed color, the variation of quantity due to the variation of the tonerlayer thickness can be reduced in exposure, thereby attaining to a clearcolor image with less color unevenness. In addition, with respect to theone-color toner image, even if the toner is partially overlapped so asto form two layers, since the saturation density is achieved with onetoner layer, it is possible to attain a high-density mixed color imagewhose color tone is stable.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematically cross-sectional view showing an arrangement ofan apparatus made by embodying a color electrophotographic methodaccording to an embodiment of the present invention.

FIG. 2 is a schematically cross-sectional view showing an arrangement ofa developing device of the same apparatus.

MOST PREFERRED EMBODIMENT OF THE PRESENT INVENTION

The present invention will be described hereinbelow with reference toembodiments. As a color toner to be used in the present invention isused any one of transparent color toners for electrophotography whichare adjusted so as to obtain the predetermined maximum image densitywith one toner layer. Attaining a desirable image density with one tonerlayer can be easily achieved by adjusting the amount of a coloringmaterial included in the toner component. In the toner features, it ispreferable to have an excellent transparency and to be a non-magnetictoner whose resistivity is above 10¹² Ω·cm. Further, the averageparticle diameter is preferable to be below 15 μm.

For reproduction of the full-color image used three kinds of toners,i.e., yellow, magenta and cyan. In this instance, the color density ofeach of the color toners is preferable to be above 0.8. More preferably,it is over 0.8 for yellow, above 1.1 for magenta, and above 1.2 forcyan. For example, in order to obtain a color density of above 0.8 withone toner with particle diameter of 10 μm, in the case that the coloringmaterial is a pigment, it is 3 to 8 weight % with respect to the tonercomponent, and in the case of a dye, it is 1 to 6 weight % with respectthereto.

In the one-time transfer color electrophotographic method, in order toform a plurality of toner images on the photoconductor, as a developingmeans to be employed is preferable a developing method in which thedeveloper does not come directly into contact with the toner image onthe photoconductor. Here, from the viewpoint of performing thedevelopment so that the toner layer thickness is thin and stable, anelectric field flying developing method which is arranged to fly thetoner with an electric field is suitable. Particularly, a direct-currentelectric field flying developing method is suitable because thedevelopment fogging due to the reverse polarity is little.

The electric field flying developing method using a one-componentdeveloper is a method in which a toner-holding device holding a tonerthin layer is disposed to face the photoconductor so that the thin layeris in no contact relation therewith and a voltage is applied to betweenthe toner-holding device and the photoconductor so as to fly the toner.Therefore, it is preferable that the toner has an excellent flowabilityand the charge amount is stable in a range of 1 to 15 μC/g. Providingsuch characteristics to the toner is achieved by keeping on the surfaceor inside the toner component an inorganic material such as silica,barium sulfate, barium titanate, aluminum oxide, titanium oxide and tinoxide. Particularly, with respect to the toner whose surface hasmicroscopic powder of silica and tin oxide, the charge amount is stableso as to attain uniform image density. The addition amount of the silicaand the tine oxide is suitable to be below 1 weight % with respect tothe toner component.

One example of arrangements of a developing device based on thedirect-current electric field flying method is illustrated in FIG. 2. InFIG. 2, numeral 13 represents a toner container, 14 designates anon-magnetic toner, 15 depicts a toner holding device constructed by acylindrical metal such as aluminum and stainless, 16 denotes aconductive fur brush roller arranged so that a conductive roller carriesa resin fiber including carbon, for example, or a metal fiber, 17represents a rubber blade, 18 designates a direct-current electricpower, and 20 is a switch. The toner holding device 15 is disposed tokeep a predetermined distance with respect to the photoconductor 19 sothat the toner does not come into contact with the photoconductor 19.The space between the toner holding device 15 and the photoconductor 19is preferable to be below 300 μm, more preferably 50 to 150 μm.

When rotating the toner holding device 15 and the fur brush roller 16 inthe direction indicated by an arrow, the toner 14 is frictionallycharged so as to be electrostatically adhered to the toner holdingdevice 15. Further, it is rolled by the rubber blade 17 so as to form atoner thin layer, whose thickness is 20 to 50 μm, on the toner holdingdevice 15. Here, it is appropriate that the fur brush roller 16 iselectrically risen or grounded. Further, for control of the imagedensity, a direct-current voltage or an alternating current voltage isapplied between the fur brush roller 16 and the toner holding device 15so as to electrically control the toner amount to be supplied to thetoner holding device 15.

Subsequently, a description will be given in terms of a techniquepreferable to reproduce a full-color image with gradation in theone-time transfer color electrophotographic method according to thepresent invention.

For the reproduction of the gradation image either well known densitygradation method or area gradation method is usable. Of these methods,suitable is the area gradation method which divides the image into aplurality of small picture elements and indicates in dummy the gradationby varying the area with the density of the picture element beingconstant. This is due to that fact that the present invention isparticularly excellent in the case of reproducing a solid image withuniform density and high density.

For obtaining a gradation image by means of the area gradation method,as a light source is used an optical writing device such as a laseroptical system, a photodiode array and a crystal liquid switchingelement and used is a well known method which performs the scanningexposure in correspondence with an image signal area-modulated.

EMBODIMENT 1

With a manufacturing method to be disclosed hereinbelow, three kinds oftoners, i.e., yellow toner, magenta toner and cyan toner can beobtained.

(1) Yellow Toner

After kneading the following components for about two hours at atemperature of 150° C., a yellow toner base material of 5 to 15 μm(average particle diameter: 10 μm) can be obtained by cooling, smashingand classification.

coloring material: C.I. pigment yellow 12--50 g

binding agent: styrene acryl resin--930 g

charge control agent: aminated styrene resin--20 g

Secondary, the following mixtures are agitated by means of a Henschelmixer to obtain a yellow toner.

Y toner base material--500 g

silica fine powder--20 g

tin oxide fine powder--15 g

(2) Magenta Toner

After kneading the following components for about two hours at atemperature of 150° C., a magenta toner base material of 5 to 15 μm(average particle diameter: 10 μm) can be obtained by cooling, smashingand classification.

coloring material: C.I. pigment red 5--60 g

binding agent: styrene acryl resin--910 g

charge control agent: aminated styrene resin--30 g

Secondary, the following mixtures are agitated by means of a Henschelmixer to obtain a magenta toner.

M toner base material--500 g

silica fine powder--20 g

tin oxide fine powder--15 g

(3) Cyan Toner

After kneading the following components for about two hours at atemperature of 150° C., a cyan toner base material of 5 to 15 μm(average particle diameter:10 μm) can be obtained by cooling, smashingand classification.

coloring material: C.I. pigment yellow 15--50 g

binding agent: styrene acryl resin--930 g

charge control agent: aminated styrene resin--20 g

Secondary, the following mixtures are agitated by means of a Henschelmixer to obtain a cyan toner.

C toner base material--500 g

silica fine powder--20 g

tin oxide fine powder--15 g

A color image has been formed using the above-mentioned three kinds oftoners by means of an apparatus illustrated in FIG. 1.

In FIG. 1, numeral 1 represents a photoconductor (the layer thickness ofthe photosensitive layer: 60 μm, electrostatic capacity: 92 pF/cm²)formed by deposition of selenium-tellurium on an aluminum drum, 2designates a scorotron charger (corona voltage: +7 kV, grid voltage:+850 V), 3 denotes a light emitting diode array (output: 7 μW, emittedlight wavelength: 670 nm, dot density: 240 dot/inch), 4 is a focusinglens array, 5, 6 and 7 represent developing devices in which yellow,magenta and cyan toners are independently encased, 8 designates anelectricity-removing device such as an erase lamp and an AC coronadischarger, 9 depicts a corona charger for transfer, 10 is an AC eraserfor paper separation, 11 depicts a plain paper sheet and 12 is acleaning brush.

Each of the developing devices has the same arrangement as in thedescription of FIG. 2. As the toner holding device 15 is used analuminum tube whose surface is roughened, and as the fur brush roller 16is used a device constructed by planting a rayon fiber including acarbon, whose resistivity is 10⁶ Ω, on an aluminum tube. The chargeamount of each toner holding device when the developing device is drivenhas been found to be 2 to 5 μC/g. Further, the space between thephotoconductor and the toner holding device is determined to be 150 μm.

A description will be made hereinbelow in terms of an image-formingmethod. With the photoconductor 1 being rotated at a speed of 100 mm/sin the direction indicated by an arrow, the photoconductor 1 is chargedup to +800 V by means of the scorotron charger 2. Subsequently, theyellow image signal is scanning-exposed by means of the light emittingdiode array 3, thereby resulting in a non-imaged line portion of +800 Vand an image portion of +40 V so as to form a negative electrostaticlatent image. After the exposure, the photoconductor 1 is passed thoughthe three developing devices so as to perform an inversion developmentwith the Y toner. The layer thickness of the Y toner developed is about12 μm. In this instance, the set conditions of the respective developingdevices are as follows.

(1) Yellow Developing Device 5

application voltage to the toner holding device: +750 V

application voltage to the fur brush: +850 V

toner layer thickness on toner holding device: about 40 μm

(2) Magenta and Cyan Developing Devices 6,7

application voltage to the toner holding device: grounded

application voltage to the fur brush: grounded

toner layer thickness on toner holding device: about 40 μm

After development, the photoconductor 1 holding the yellow toner imageis illuminated by means of the erase lamp 8, and after light dischargefor the electrostatic latent image, it is again charged by the scorotroncharger 2. The surface potential of the photoconductor 1 is +800 Virrespective of the presence or absence of the toner.

Subsequently, the magenta image signal is scanning-exposed by means ofthe light emitting diode array 3 so as to form a negative electrostaticlatent image. The surface potential of the image portion at a portionthat the yellow toner is absent is +40 V, and the surface potential ofthe image region at the yellow-toner attaching portion is +160 V.Following the exposure, the photoconductor 1 is passed through the threedeveloping devices 12, 13 and 14 under the following conditions toperform the inversion development with the magenta toner. The layerthickness of the toner image obtained is about 12 μm at the portion thatonly the magenta toner is present and is about 21 μm at the portion thatthe yellow toner and the magenta toner are overlapped with each other.The magenta toner is not adhered at all at the non-image region in the Ytoner attachment portion.

(1) Yellow and Cyan Developing Devices 5, 7

application voltage to the toner holding device: +750 V

application voltage to the fur brush: +550 V

toner layer thickness on toner holding device: 0

(2) Magenta Developing Device 6

application voltage to the toner holding device: +750 V

application voltage to the fur brush: +850 V

toner layer thickness on toner holding device: about 40 μm

After again electricity-removing the photoconductor 1, it is charged bythe scorotron charger 2. the surface potential of the photoconductor is+800 V regardless of the presence or absence of the toner.

Secondly, the cyan image signal is scanning-exposed by means of thelight emitting diode array 3. The surface potential of the image portionat a portion that the toner is absent is +40 V, the surface potential is+160 V at the portion that only the yellow toner and the magenta tonerare adhered, and the surface potential is +220 V at the portion that theyellow toner and the magenta toner are overlapped with each other. Thephotoconductor 1 is passed through the three developing devices 5, 6 and7 under the following conditions to perform the inversion developmentwith the cyan toner. The cyan toner is not adhered at all at thenon-image region in the yellow and magenta toner attachment portion.

(1) Yellow and Magenta Developing Devices 5, 6

application voltage to the toner holding device: +750 V

application voltage to the fur brush: +550 V

toner layer thickness on toner holding device: 0

(2) Cyan Developing Device 6

application voltage to the toner holding device: +750 V

application voltage to the fur brush: +850 V

toner layer thickness on toner holding device: about 40 μm

Further, after illuminating the entire surface of the photoconductor 1by means of the erase lamp 8, the toner image on the photoconductor 1 istransferred to a plain paper sheet 11 by means of the corona charger 9(corona voltage: -5.5 kV) and then the plain paper sheet 11 iselectricity-removed by the AC eraser 10 and separated from thephotoconductor 1. The toner image transferred to the plain paper sheet11 is heated by a heat fixing device (not shown) so as to attain a colorprint. Following the transfer, the remaining toner on the photoconductor1 is removed by the cleaning brush 12 so that the photoconductor 1 isagain placed in condition for allowing the next image formation. As aresult, obtained is a clear color print in which the respectivereproduced color densities are as follows: yellow:1.1, magenta:1.4,cyan:1.5, red: 1.4, green:1.4, blue-violet:1.5, and black:1.4 resultingfrom three-color composition.

EMBODIMENT 2

A description will be described hereinbelow in terms of a method ofobtaining a full-color image by the area gradation method due to thewell known Dither matrix technique using the apparatus described in theembodiment 1. For the respective image signals of yellow, magenta andcyan, the Dither process is performed so that one picture element takes4 dot×4 dot, and with the method similar to that of the embodiment 1,the color image is reproduced, whereby it is possible to obtain afull-color image so that each of yellow, magenta and cyan has 16gradations.

INDUSTRIAL APPLICATION POSSIBILITY

As described above, according to the present invention, since it ispossible to obtain a clear mixed color image with high density andstable color tone, it is suitable for a color hard copying apparatussuch as color copying machine, color light printer and color facsimile.

What is claimed is:
 1. A color electrophotographic method of using atleast two different toners and performing a charge, exposure anddevelopment so as to superimpose different-color toner images on oneelectrophotographic photoconductor, said method comprising the stepsof:(a) uniformly charging said electrophotographic photoconductor bymeans of a corona charger; (b) illuminating said photoconductor with afirst light-image so as to form a first electrostatic latent image onsaid photoconductor; (c) applying a direct-current voltage between saidphotoconductor and a developer-holding device which carries a thin layerof a first one-component developer mainly comprising a first coloredpowder toner and which is disposed to be in confronting relation to saidphotoconductor with a predetermined separation being maintainedtherebetween so as not to cause said developer to directly come intocontact with said photoconductor whereby said developer on said holdingdevice moves toward said photoconductor by means of the action of anelectric field so that said first electrostatic latent image formed onsaid photoconductor is developed with said first one-component developerhaving the colored first powder toner to form a first toner image bysaid first powder toner on said photoconductor; (d) again uniformlycharging said photoconductor holding said first toner image from anupper side of said first toner image by means of a corona charger; (e)illuminating said photoconductor holding said first toner image with asecond light-image so as to form a second electrostatic latent imagethereon; and (f) developing said second electrostatic latent image usinga second one-component developer comprising a second colored powdertoner in accordance with a developing method as described in step (c) soas to superimpose a second toner image of said second colored powdertoner on said photoconductor holding said first toner image, whereineach of said first and second one-component developers are non-magneticmaterials mainly composed of a light-transmitting non-magnetic tonermainly comprising a resin binding agent, at least one of a dye andcolored pigment, and a charge control agent and having an averageparticle diameter below 15 μm, and in steps (c) and (f) said developmentis performed under the conditions that the toner charge amount of saiddeveloper thin-layered on said developer-holding device is 1 to 15 μC/g,the separation between said developer-holding device and saidphotoconductor is below 300 μm, and the value of the direct-current tobe applied to between said developer-holding device and saidphotoconductor is determined so that the maximum thickness of the tonerto be electrostatically adhered onto said photoconductor though thedevelopment is below twice the average particle diameter of the toner.2. A color electrophotographic method as claimed in claim 1, wherein toeach of said first and second one-component developers there is added atleast one inorganic fine powder selected from a group of silica, bariumsulfate, barium titanate, titanium oxide and tin oxide.
 3. A colorelectrophotographic method as claimed in claim 2, wherein to each ofsaid first and second one-component developers there are added thesilica and tin oxide each of whose amount is below 1 weight % withrespect to the toner.
 4. A color electrophotographic apparatuscomprising:an endless electrophotographic photoconductor arranged to bemovable in one direction; a charger for applying a charge to saidphotoconductor; a light-writing light source for performingimage-exposure in correspondence with an image signal; a plurality ofdeveloping devices independently having one-component developers mainlycomprising yellow, magenta and cyan powder toners, respectively, each ofsaid developing devices being equipped with developer-holding meansholding thereon a thin layer of the corresponding powder toner which isdisposed in confronting relation to said photoconductor with apredetermined separation maintained therebetween so as not to cause thethin-layer toner to come into contact with said photoconductor, andfurther equipped with voltage-applying means for applying adirect-current voltage between said developer-holding means and saidphotoconductor so as to cause toner to fly toward said photoconductor bymeans of an electric field to form a toner image on said photoconductor;and a transferring device for electrostatically transferring the formedtoner image to an image-receiving sheet, said apparatus moving saidphotoconductor to uniformly charge said photoconductor by said charger,and exposing said photoconductor with light emitted from said lightsource in accordance with the selected one of said image signals for theyellow, magenta and cyan toners so as to form an electrostatic latentimage on said photoconductor, and operating the corresponding developingdevice to develope the formed electrostatic latent image so as to formthe toner image on said photoconductor, said apparatus furtherperforming the similar processes for forming the other color tone imagesone upon another on said photoconductor to produce a superimposed colortoner image, and performing batch transfer of the superimposed colortoner image onto the image-receiving sheet by means of said transferringdevice, wherein said one-component developer for each of the yellow,magenta and cyan is mainly composed of a light-transmitting non-magnetictoner mainly comprising a resin binding agent, at least one of a dye andcolored pigment, and a charge control agent and having an averageparticle diameter below, 15 μm, and said development is performed underthe conditions that the toner charge amount of said developerthin-layered on said developer-holding device is 1 to 15 μC/g, theseparation between said developer-holding device and said photoconductoris below 300 μm, and the value of the direct-current to be applied tobetween said developer-holding device and said photoconductor isdetermined so that the maximum thickness of the toner to beelectrostatically adhered onto said photoconductor through thedevelopment is below twice the average particle diameter of the toner.5. A color electrophotographic apparatus as claimed in claim 4, whereineach of said developing devices includes said developer-holding meanshaving an endless configuration movable so as to carry and providedeveloper up to and at the vicinity of said photoconductor, a developervessel for encasing the developer, developer-supplying means forsupplying the developer from said developer vessel up to a surface ofsaid developer-holding means, and layer-thickness limiting means forforming a thin layer of the developer held on the surface so that thethickness of the formed thin layer becomes constant.
 6. A colorelectrophotographic apparatus as claimed in claim 4, wherein said imagesignal is a signal area-modulated.